1. Field of the Invention
The present invention relates to an apparatus for controlling a laser diode irradiating a light beam on an optical recording medium.
2. Description of Background Information
In case of recording, reproducing, and erasing information by irradiating light beam to an optical magnetic recording medium such as a magento-optic disc or the like, a stronger light beam is needed for recording an erasing information rather than in reproduction of recorded information from the optical magnetic recording medium. Hitherto, an output adjustment of such a light beam is performed by an APC (Automatic Power Control) operating in such a manner that a part of the light beam generated from a laser diode is detected by a photodetector and the driving of the laser diode is controlled in accordance with a detection output.
According to the conventional control method as mentioned above, however, when the detection output according to the light beam is not obtained from the photodetector due to dew condensation, a failure, or the like, it is judged that the light beam has been reduced, thereby performing a control operation to raise the output of the laser diode. As a result of the control operation, the laser diode will irradiate a stronger laser beam on the optical magnetic recording medium, which in turn create a problem that the information recorded on the optical magnetic recording medium is erased even during a period in which recorded data is being reproduced.
A laser diode drive control apparatus shown in FIG. 1 has recently been devised with an aim to solve such a problem of the prior art. In such an apparatus, a light beam for recording and reproducing information is generated by a laser diode 1. An optical system comprising a collimating lens 4, a beam splitter 5, and an objective lens 6 is provided to lead the light beam to an optical recording medium 3 which is rotated by a spindle motor 2. The apparatus also includes recording information signal detecting means 7 for receiving the light beam reflected from the optical recording medium 3 and for detecting the recording information signal of the optical recording medium 3. The laser diode 1 irradiates the beam in two, namely first and second, directions. The beam in the first direction passes through the collimating lens 4 and beam splitter 5 and enters a first photodetector 8. The first photodetector 8 transmits an output according to a light reception amount to a first comparator 9 and a second comparator 10. The first comparator 9 sends a comparison output corresponding to a difference between the output from the photodetector 8 and a reference input REF to a control circuit 11. The beam in the second direction from the laser diode 1 enters a second photodetector 12. The second photodetector 12 sends an output according to a light reception amount to the second comparator 10 in a manner similar to the first photodetector 8. The second comparator 10 compares the absolute values of the outputs of the first and second photodetectors 8 and 12 and generates a positive voltage corresponding to a difference between them to the control circuit 11. The control circuit 11 gives a driving current according to the output of the first comparator 9 to the laser diode 1, thereby controlling the driving of the laser diode 1 so as to irradiate the light beam of a proper intensity to the optical recording medium 3. The control circuit 11, further, detects whether or not the value of the positive voltage of the second comparator 10 lies within a predetermined range. When the positive voltage value is equal to or larger than a predetermined value, the control circuit 11 generates an abnormality detection signal by judging that an abnormality has occurred in the optical transmission system including the first and second photodetectors 8 and 12. Simultaneously with the generation of the abnormality detection signal, the control circuit 11 shuts off the driving current being supplied to the laser diode 1, thereby stopping the driving of the laser diode 1.
In case of control operation which is performed by detecting the light beams in two directions by means of two separate photodetectors as in the apparatus described above, however, there is a problem that output characteristics of the light beam are changed due to an influence by the return light, so that an accurate control operation cannot be performed.